Since the issuance of U.S. Pat. No. 3,028,365 in April of 1962, aromatic polycarbonate has become well known and accepted as a thermoplastic resin suitable for a wide variety of uses including injection molding, extrusion and film formation. The chemistry, synthesis, properties and applications of these polycarbonates are extensively discussed in Chemistry and Physics of Polycarbonates by Schnell, Interscience, 1964 and Polycarbonates by Christopher and Fox, Reinhold, 1962.
Although polycarbonates have some inherent flame resistance, being self-extinguishing, ever more demanding flame retardancy requirements have spawned numerous attempts to increase this property. Two general approaches have been followed.
One approach has been to add substantial amounts of halogen, particularly bromine or chlorine, to polycarbonate compositions. The halogen can be carried by polycarbonate polymer chains as in U.S. Pat. Nos. 3,751,400 and 3,334,154 or by a monomeric compound as in U.S. Pat. No. 3,382,207. However, the presence of substantial amounts of halogen has been found to be detrimental to the properties of the polycarbonate and numerous additives such as those proposed in U.S. Pat. Nos. 3,647,747 and 3,733,295 have been proposed to overcome these detrimental effects.
Flame retardant polycarbonate compositions containing polytetrafluoroethylene (PTFE) are known in the art. U.S. Pat. No. 4,391,935 discloses such compositions which also include certain salts. U.S. Pat. No. 3,294,871 discloses a process for producing molding compositions containing PTFE and a thermoplastic polymer entailing blending the thermoplastic polymer with a latex of the PTFE to produce a dispersed, substantially homogeneous mixture of the PTFE and a thermoplastic polymer and recovering the composition by removing the volatiles therefrom. Among the polymers polycarbonates are listed at col. 3, lines 6-8. The disclosed compositions are said to offer advantages in terms of impact strength, melt index, coefficient of friction, low water permeability and increased HDT.
U.S. Pat. No. 3,005,795 discloses compositions containing PTFE resin in a form of finely divided microfibrous and submicrofibrous particles. Polycarbonates are disclosed at col. 3, lines 18-19 as suitable matrix. Improved moldability of PTFE resins is disclosed in U.S. Pat. No. 3,290,412 to result upon adding thereto of 10 to 40% of polycarbonates.
U.S. Pat. No. 3,723,373 discloses adding PTFE emulsion to polyethylene terephthalate for improved processability. French Patent No. 1,412,767 addresses problems associated with the moldability of PTFE.
Lontz et al in Industrial and Engineering Chemistry, Vol.44, No. 8 (1952) at page 1800 describes the general dispersion properties in film forming characteristics of PTFE. Coagulation of the dispersion by the addition of electrolytes or other solvents or by other means is also disclosed.
German published specification DE-OS 33 22 260 discloses PTFE co-precipitated with ABS and added to polycarbonate. Related technologies are disclosed in EP-OS 0,154,138 which disclosed a system entailing ABS and PTFE, DE-OS 34 17 476 which discloses the system of polyamides and PTFE and DE-OS 34 18 750 which concerns the system of ABS co-precipitated with PTFE. Also of relevance are DE-OS 34 20 002, DE-OS 34 22 862, DE-OS 35 12 638 (equivalent to EP-OS 0,174,493), DE-OS 29,48 439 and U.S. Pat. No. 4,208,489.
U.S. Pat. No. 3,940,455 discloses the incorporation of Teflon.RTM. in elastomeric hydrocarbons by dissolving the elastomeric polymer in a solvent and incorporating the PTFE to form a suspension and subsequently co-precipitating both by mixing the suspension with a non-solvent. The co-precipitate is said to exhibit improved physical properties.
Also relevant in the present context is U.S. Pat. No. 4,753,994 which disclosed flame retardant polycarbonate compositions containing a co-precipitate of polycarbonate and fluoropolymer. Also relevant is EP 899 303 for its disclosure of a flame retardant polycarbonate composition which contains fluoropolymer resin particles that are encapsulated by thermoplastic SAN copolymer. U.S. Pat. No. 5,773,493 is noted here for its disclosure of a method of dispersing additives such as PTFE in a thermoplastic polymer and the use of the product of the inventive method in preparing molding compositions.